Powder metal with attached ceramic nanoparticles

ABSTRACT

A powder material includes spherical metal particles and a spaced-apart distribution of ceramic nanoparticles attached to the surfaces of the particles.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. patent application Ser. No.15/678,260 filed Aug. 16, 2017, which is a continuation of U.S. patentapplication Ser. No. 14/670,623 filed Mar. 27, 2015.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with government support under contract numberW911NF-14-2-0011 awarded by the United States Army. The government hascertain rights in the invention.

BACKGROUND

High performance alloys can be used in relatively severe environments toprovide enhanced mechanical properties, such as high strength, creepresistance, and oxidation resistance. For example, such alloys aredispersion-strengthened and include a metallic matrix with a secondphase of oxide, nitride, or carbide dispersed uniformly throughout thematrix.

One technique for fabricating dispersion-strengthened alloys is milling.Milling involves ball milling a metal feedstock powder and reinforcementphase particles to incorporate the reinforcement phase particles intothe metal powder. The reinforcement phase particles are generally notsoluble in the base metal. Long times are needed to achieve anappropriate dispersion and process control agents are often needed tolimit agglomeration. The agents must later be removed, the resultingparticles are irregularly-shaped, and there is also difficulty inachieving consistency from batch-to-batch.

SUMMARY

A powder material according to an example of the present disclosureincludes spherical metal particles and a spaced-apart distribution ofceramic nanoparticles attached to the surfaces of the particles.

In a further embodiment of any of the foregoing embodiments, thespherical metal particles are selected from the group consisting ofnickel, chromium, and combinations thereof.

In a further embodiment of any of the foregoing embodiments, the ceramicnanoparticles are selected from the group consisting of oxides,nitrides, carbides, and combinations thereof, and the powder materialhas a composition, by weight, of 0.1-5% of the ceramic nanoparticles.

In a further embodiment of any of the foregoing embodiments, the ceramicnanoparticles are oxide nanoparticles.

In a further embodiment of any of the foregoing embodiments, the ceramicnanoparticles are zirconium oxide nanoparticles.

In a further embodiment of any of the foregoing embodiments, thespherical metal particles are nickel-based particles and includechromium.

In a further embodiment of any of the foregoing embodiments, the ceramicnanoparticles are zirconium oxide nanoparticles.

In a further embodiment of any of the foregoing embodiments, the powdermaterial has a composition, by weight, of 0.1-5% of the ceramicnanoparticles.

BRIEF DESCRIPTION OF THE DRAWINGS

The various features and advantages of the present disclosure willbecome apparent to those skilled in the art from the following detaileddescription. The drawings that accompany the detailed description can bebriefly described as follows.

FIG. 1 illustrates an example method for processing a powder material.

FIG. 2 illustrates a low magnification micrograph of metal particlesthat have ceramic nanoparticles attached on the surfaces.

FIG. 3 illustrates a high magnification micrograph of the surface of ametal particle with a spaced-apart distribution of ceramic nanoparticlesattached on the surface.

DETAILED DESCRIPTION

FIG. 1 illustrates an example method 20 for processing a powdermaterial. As will be described, the resulting powder material producedusing the method 20 has spherical metal particles with a spaced-apartdistribution of ceramic nanoparticles attached on the surfaces of thespherical metal particles. The powder material can readily be used in anadditive fabrication process to form an end-use article withdispersion-strengthening from the ceramic nanoparticles.

Alloys with dispersed secondary reinforcement phases can be fabricatedby milling; however, this processing technique cannot produce powderthat can be used in additive fabrication processing. Powders in additivefabrication processes are fed through an additive manufacturing machineand deposited layer-by-layer in a workspace where an energy beam can beused to selectively fuse portions of the layers to form the end-usearticle. For proper feeding and deposition of the layers, the powdertypically has a controlled particle size and a spherical powder particleshape that permits easy flow through the equipment and uniformdeposition of the layers. Thus, although dispersion-strengthened alloyscan be fabricated by milling, the resulting particles areirregularly-shaped and are thus not suited for reliable flow throughadditive fabrication equipment. The example method 20 provides aspherical metal powder that has ceramic nanoparticles attached to thesurfaces thereof and which can be readily used in additive fabricationprocessing.

As will be appreciated, the steps of the method 20 can be used incombination with other processing steps. The example method 20 includesa cleaning step 22. In the cleaning step 22, the surfaces of an initialpowder material are prepared for attachment of the ceramicnanoparticles. The initial powder material includes spherical metalparticles. For example, the spherical metal particles have an averageparticle size of approximately 10-50 micrometers, which will typicallybe suitable for many additive fabrication techniques. Of course, thepowder may have a different average size if needed by a particularadditive fabrication process.

The metal can be a pure metal or an alloy of several metals. Althoughnot limited, the metal can include nickel, chromium, aluminum, titanium,iron, or combinations thereof, which may be useful in aerospacearticles.

The surfaces of the initial powder material may contain oxides and/orforeign substances that can otherwise inhibit attachment of the ceramicnanoparticles. For example, the initial powder material can be cleanedusing an acid, to etch away surface oxides and foreign substances toprovide a “fresh” metal surface for attachment. The type andconcentration of the acid can be selected in accordance with the metalor metals to effectively etch the surfaces without damaging the bulkparticles.

The cleaned spherical metal particles are then subjected to a coatingstep 24. In the coating step 24, the cleaned spherical metal particlesare coated with an organic bonding agent. The organic bonding agent willlater facilitate attachment of the ceramic nanoparticles in the method20.

As an example, the organic bonding agent includes a surfactant that haspolar end groups. The polar end groups facilitate polar bonding with thesurfaces of the spherical metal particles and, later in the method 20,also polar bonding with the ceramic nanoparticles. In this regard, thesurfactant can be selected in correspondence with the metal of thespherical metal particles and the composition of the ceramicnanoparticles such that polar end groups are selected for polar bondingwith the metal particles and also with the ceramic nanoparticles. Forexample, the metal and the ceramic nanoparticles may have either apositive polarity or a negative polarity, and the end groups areselected to have a negative or positive polarity to form polar bondswith the metal and with the ceramic nanoparticles.

In a further example, the surfactant includes dodecyl sulfate, such assodium dodecyl sulfate. For instance, the dodecyl sulfate can be usedwith metal particles that include nickel, chromium, or combinationsthereof and with oxide ceramic nanoparticles.

After the coating step 24, the coated spherical metal particles aresubjected to a mixing step 26. In the mixing step 26, the coatedspherical metal particles are mixed with a dispersion that contains acarrier substance, such as a liquid-based medium, and the ceramicnanoparticles. The ceramic nanoparticles can include oxide particles,nitride particles, carbide particles, or mixtures thereof. Zirconiumoxide is one example of oxide particles. Silicon carbide is one exampleof carbide particles. Silicon nitride is one example of nitrideparticles.

The dispersion can include, but is not limited to, a colloid that has asuspension of the ceramic nanoparticles in the carrier substance. Themixture can be agitated or stirred for a period of time to disperse theceramic nanoparticles uniformly over the surfaces having the organicbonding agent. The ceramic nanoparticles attach by polar bonding to thepolar end groups of the organic bonding agent.

The spherical metal particles can then be rinsed in water. The rinsingremoves much of the excess dispersion and carrier substance. Since theceramic nanoparticles are relatively weakly bonded by polar bonding(e.g., by van der Waals forces) to the organic bonding agent, severerinsing with large amounts of water and agitation may undesirably washaway some of the bonded ceramic nanoparticles. Thus, in one example, acontrolled amount of deionized water and, optionally, gentle stirring,can be used for the rinse to limit wash-away loss of the bonded ceramicnanoparticles.

For example, the controlled amount of water can be a function of theconcentration of the ceramic nanoparticles in the dispersion, theconcentration of the ceramic nanoparticles bonded on the spherical metalparticles, or both. Thus, only a limited amount of water may be used toavoid washing away a substantial amount of the bonded ceramicnanoparticles and to produce a desired spaced-apart distribution of theceramic nanoparticles. Given this disclosure, those skilled in the artwill be able to readily determine appropriate rinsing throughexperimentation using different amounts of water and observation of howmuch of the ceramic nanoparticles are washed away.

Residual amounts of the carrier substance may be present on thespherical metal particles, even after washing. At step 28, the sphericalmetal particles are dried to remove any residual carrier and to depositthe ceramic nanoparticles with the spaced-apart distribution onto theorganic bonding agent on the surfaces of the spherical metal particles.For example, the spaced-apart distribution of the ceramic nanoparticlesis provided by the selected concentration of the ceramic nanoparticlesin the dispersion that is used in the mixing step 26. If thisconcentration is relatively high, the ceramic nanoparticles will bedeposited in a continuous coating rather than in the spaced-apartdistribution. The spaced-apart distribution is desired for providing adispersion of the ceramic nanoparticles in the end article afteradditive fabrication, whereas a continuous coating may result inagglomeration of the ceramic nanoparticles.

At step 30, the organic bonding agent is thermally removed from thespherical metal particles to thereby attach the ceramic nanoparticles tothe surfaces of the spherical metal particles. For example, thespherical metal particles are thermally treated in a heating chamber atan elevated temperature for a determined period of time to thermallyremove the organic bonding agent. As can be appreciated, the specifictreatment temperature may be dependent upon the selected organic bondingagent. However, in most instances, organic materials will decompose andvolatilize from the powder at temperatures above approximately 550° C.in an air or an inert environment.

The resulting powder material includes the spherical metal particleswith the ceramic nanoparticles attached, by polar bonding, to thesurfaces thereof. In further examples, the resulting powder material hasa composition, by weight, of 0.1-5% of the ceramic nanoparticles and aremainder of the metal. In a further example, the spherical metalparticles include nickel and chromium, the ceramic nanoparticles arezirconium oxide, and the resulting powder material has a composition, byweight, of 0.1-5% of the zirconium oxide. The amount of ceramicnanoparticles on the resulting powder material can be controlled bycontrolling the concentration of the ceramic nanoparticles in step 26and the optional controlled rinsing. When the content of ceramic coatingis below a desirable amount in one single coating process describedabove, the coating process can be repeated from the step of adding theorganic bonding to the step of thermal treatment until the desirablecontent is achieved.

FIG. 2 shows a micrograph at low magnification of several sphericalmetal particles 30 that have the ceramic nanoparticles 40 attached onthe surfaces thereof. FIG. 3 shows a representative surface of one ofthe spherical metal particles with the spaced-apart distribution of theceramic nanoparticles 40 (whitish in color) attached on the surfacethereof (dark color). As shown, the ceramic nanoparticles 40 arediscrete particles on the surface, with a relatively uniform,spaced-apart distribution.

The following is a further, non-limiting example of the method 20.Nanoparticles of zirconium oxide (ZrO₂) were attached onto surfaces of aspherical metal powder of composition nickel-20 wt % chromium usingsodium dodecyl sulfate as the surfactant organic bonding agent. Thezirconium oxide was provided in a colloidal solution with nanoparticlesizes of approximately five to ten nanometers. Approximately 30 grams ofthe Ni-20 wt. % Cr powder with a mean particle size of approximately 40micrometers was etched in a beaker using approximately 14 ml of 18.5 wt% hydrochloric acid for several minutes to create a fresh surface on thepowder. The etched powder was rinsed with deionized water several times.Approximately 75 ml of 1 wt % sodium dodecyl sulfate was added into thebeaker with the etched powder and was stirred at room temperature forseveral hours. The powder was then rinsed with deionized water severaltimes and dried in an oven at 120° C. for several hours. Approximately21 grams of 20 wt % zirconium oxide colloid solution was added into thebeaker and the powder was stirred at approximately 40° C. for severalhours, followed by an aging process at room temperature forapproximately one day. The powder was then rinsed with a controlledamount of deionized water, to avoid wash-away of the attached zirconiumoxide nanoparticles. The powder was then dried in an oven atapproximately 120° C. for several hours with a ramp rate ofapproximately 2° C./min. The dried powder was then calcined in a furnaceat 550° C. with a ramp rate of 2° C./min for several hours under ambientatmosphere. The zirconium oxide nanoparticles were observed in aspaced-apart distribution on the surface of the powder.

The powder material produced from the method 20 may be further processedin another method that includes feeding the powder through an additiveprocessing machine to deposit multiple layers of the powder materialonto one another, and then using an energy beam to thermally fuseselected portions of the layers to one another with reference to data,such as computer-aided drawing data, relating to a particularcross-section of an article being formed. The energy beam melts orpartially melts the metal such that the selected portions of the layersof powder fuse together. The ceramic nanoparticles may not melt, butdisperse into the melted or softened portions of the metal to thusprovide a relatively uniform dispersion of reinforcement through theformed article.

Although a combination of features is shown in the illustrated examples,not all of them need to be combined to realize the benefits of variousembodiments of this disclosure. In other words, a system designedaccording to an embodiment of this disclosure will not necessarilyinclude all of the features shown in any one of the Figures or all ofthe portions schematically shown in the Figures. Moreover, selectedfeatures of one example embodiment may be combined with selectedfeatures of other example embodiments.

The preceding description is exemplary rather than limiting in nature.Variations and modifications to the disclosed examples may becomeapparent to those skilled in the art that do not necessarily depart fromthis disclosure. The scope of legal protection given to this disclosurecan only be determined by studying the following claims.

What is claimed is:
 1. A powder material comprising: spherical metalparticles; and a spaced-apart distribution of ceramic nanoparticlesattached to the surfaces of the particles.
 2. The powder material asrecited in claim 1, wherein the spherical metal particles are selectedfrom the group consisting of nickel, chromium, and combinations thereof.3. The powder material as recited in claim 1, wherein the ceramicnanoparticles are selected from the group consisting of oxides,nitrides, carbides, and combinations thereof, and the powder materialhas a composition, by weight, of 0.1-5% of the ceramic nanoparticles. 4.The powder material as recited in claim 1, wherein the ceramicnanoparticles are oxide nanoparticles.
 5. The powder material as recitedin claim 1, wherein the ceramic nanoparticles are zirconium oxidenanoparticles.
 6. The powder material as recited in claim 1, wherein thespherical metal particles are nickel-based particles and includechromium.
 7. The powder material as recited in claim 6, wherein theceramic nanoparticles are zirconium oxide nanoparticles.
 8. The powdermaterial as recited in claim 7, wherein the powder material has acomposition, by weight, of 0.1-5% of the ceramic nanoparticles.